I received a request to fit another of my maps to align with the Roll20 grid. Like many of my older maps, this one was created long before Roll20 existed so there was no consideration for making it match a snap-to-grid. This one was even more off as it is 3 maps stitched together with no thought to having them line up with each other.
To get this to align on Roll20: create a new map space that is 51 inches wide by 40 inches high. Then set the upper left corner of the map into the upper left corner of the grid, grab the lower right corner of the map and stretch it down into the lower right corner of the grid. Note: The 1/2 inch white border on the right and bottom sides is part of that 51x40 and should help with the alignment.
There are a couple rooms that will still not match the grid because they use a floor tile that is not set on the same grid. There's nothing I can do about it, but at least the rooms are not to big. And there is always the option of turning off the snap to grid, Roll20 plays better without it.